This invention relates to a fuel pump for use in supplying fuel under high pressure for injection into a combustion space of an associated compression ignition internal combustion engine. The invention is particularly applicable to unit pump/injectors or to unit pumps, each being intended to deliver fuel to a single associated fuel injector. It will be appreciated, however, that the invention is also applicable to other types of fuel pump, for example in-line pumps.
The pump of a unit pump/injector typically comprises a pumping plunger reciprocable within a bore under the action of a cam drive arrangement. Motion of the plunger, in use, is dependent upon the cam profile and the speed of operation of the cam drive arrangement. It is desirable to be able to modify the operation of the pump, for example to permit the timing of the delivery of fuel to be controlled, thereby permitting a reduction in the levels of white smoke, particulate emissions and noise produced when the engine is cold, and reducing the risk of miss-fires.
According to the present invention there is provided a fuel pump comprising a pumping plunger reciprocable within a plunger bore under the action of a cam drive arrangement, the cam drive arrangement comprising first and second surfaces defining therebetween a chamber, the volume of which can be controlled to control the spacing of the first and second surfaces, thereby permitting control of the axial length of the cam drive arrangement.
Where the pump is of the type in which a filling port is provided, the filling port being obscured, in use, by the pumping plunger, adjustment of the axial length of the drive arrangement permits the timing at which the filling port is closed by the pumping plunger, and hence the timing of commencement of pressurisation of fuel by the pump, to be controlled. By controlling the timing of commencement of pressurisation of fuel, depending upon the nature of the fuel injector with which the pump is to be used, the timing of commencement of fuel injection can be controlled. As a result, an arrangement is possible in which the timing of fuel delivery can be advanced, for example when the associated engine is cold.
The first surface is conveniently defined by a tappet member which acts to transmit a force from the cam drive arrangement to the pumping plunger.
The second surface may be defined by a piston member slidable within a tappet bore formed in the tappet member.
Preferably, the fuel pump comprises a spring arrangement arranged to apply a biasing force to the piston member which tends to minimise the volume of the chamber.
Conveniently, the plunger bore defines a pumping chamber for fuel, reciprocal movement of the pumping plunger within the plunger bore causing fuel pressurisation within the pumping chamber, in use. The fuel pump may further comprise a further spring arrangement which is arranged to urge the tappet member in a direction to withdraw the pumping plunger from the plunger bore, thereby tending to maximise the volume of the pumping chamber.
The cam drive arrangement may include a drive member which carries the piston member, a force from the piston member being transmitted to the pumping plunger through the drive member. The drive member and the piston member may be integrally formed.
In an alternative embodiment, the piston member may be secured directly to the pumping plunger.
The tappet member may be shaped to define, at least in part, a relief passage which communicates with the chamber upon movement of the piston member beyond a predetermined position.
Alternatively, the tappet member may be provided with a circlip which is engageable with the piston member upon movement of the piston member beyond a predetermined amount.
Preferably, the tappet member is slidable within a further bore provided in a housing, the fuel pump preferably comprising means for substantially preventing angular movement of the tappet member within the further bore.
For example, one of the tappet member and the housing may be provided with a member which extends into a recess provided in the other of the tappet member and the housing so as to substantially prevent angular movement of the tappet member within the further bore.
The member may take the form of a peg carried by the tappet member, the peg extending through an opening provided in a drive member forming part of the drive arrangement so as to substantially prevent angular movement of the tappet member within the further bore.
Alternatively, the pump may comprise a yoke secured to the housing, the yoke having at least one projection extending into the further bore provided in the housing, the or each projection cooperating with the tappet member so as to substantially prevent angular movement of the tappet member within the further bore.
The piston member may include a region of part-spherical form which is cooperable with the tappet bore to permit axial misalignment between the piston member and the tappet member.
Preferably, the chamber is arranged to receive fluid through a supply passage, fluid pressure within the chamber applying a force to the first and second surfaces which serves to increase the volume of the chamber.
The fuel pump may further comprise a temperature sensitive valve arrangement, the pressure of fluid supplied to the chamber being controlled in response to an output from the temperature sensitive valve arrangement.
According to a second aspect of the present invention, there is provided an arrangement for use in a fuel pump comprising a pumping plunger which is reciprocable within a plunger bore under the action of a drive arrangement and a tappet member which is slidable within a further bore provided in a housing for transmitting a force from the drive arrangement to the pumping plunger, the arrangement comprising a yoke secured to the housing, the yoke having at least one projection extending into the bore provided in the housing, the or each projection cooperating with the tappet member so as to substantially prevent angular movement of the tappet member within the further bore.
It will be appreciated that this aspect of the invention is not limited to use in a fuel pump driven by means of a cam drive arrangement, nor is it limited to use in a fuel pump in which first and second surfaces of the drive arrangement define a chamber, the volume of which can be controlled to control the spacing of the first and second surfaces.